Thin layers, intense congregations of phytoplankton that can extend horizontally for kilometers, can be either a boon or a bust to marine food webs. On the one hand, these layers can stimulate the food web from the bottom up by providing elevated concentrations of marine snow (e.g., protozoa and organic detritus), bacteria, and plankton. On the other hand, because many of the phytoplankton species found in thin layers can be toxic, these layers can disrupt grazing, cause zooplankton and fish die-offs, and seed algal blooms at the ocean’s surface that can generate red tides. Understanding the processes driving the formation of thin layers is crucial for predicting their occurrence and ecological impact.
Although thin layer formation was previously thought to be solely influenced by abiotic forces, a recent paper in Science by William M. Durham and colleagues suggests that plankton’s swimming and shape play a role. Many phytoplankton species swim upward against gravity. When the water is calm, they swim up in a straight path. But add ocean currents to the equation, and the plankton start to encounter vertical shear where layers of faster- and slower- moving water meet. These shear forces can cause the plankton to tumble and spin instead of swimming straight up. The tumbling plankton become trapped in these regions of high shear, accumulating in a thin layer. The strength of the shear forces interacts with the morphology of the plankton to determine which species get trapped. For instance, bottom heavy species require higher shear to knock them off their straight path. Durham et al.’s findings suggest that vertical shear and cell morphology could be important predictors of red tides.
W. M. Durham, J. O. Kessler, R. Stocker (2009). Disruption of Vertical Motility by Shear Triggers Formation of Thin Phytoplankton Layers Science, 323 (5917), 1067-1070 DOI: 10.1126/science.1167334